Windows/Mac/iPhone/iPod touch: This morning Google Earth went to the Moon in celebration of the 40th anniversary of the lunar landing, but if you'd prefer to stargaze on the go, free-for-today-only iPhone application SkyVoyager and its discounted desktop counterparts are great downloads.

Windows/Mac/Linux: Google's celebrating the 40th anniversary of the Apollo moon landing today…
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SkyVoyager for the iPhone is a planetarium in your pocket, featuring over 300,000 stars and 30,000 "deep sky objects". It integrates with your iPhone's GPS to offer you an accurate version of the night sky where you're located, displays information on any star or object you tap on, and comes packed with incredible images of said objects.

If you love the look of SkyVoyager on the iPhone but want that experience on your desktop, you can also download the desktop Windows and Mac versions for a measly penny (again, today only). The site's hosting provider seems to be having some difficulties honoring the desktop downloads at the time being, so your mileage may vary on those versions.